A seat belt retractor typically comprises a rotatable spool on which a seat belt webbing is wound, a crash sensor and a locking mechanism activated in response to the crash sensor to lock the spool against rotation and thus against webbing payout. The crash sensor typically causes a locking pawl to pivot into engagement with a toothed ratchet wheel attached to the spool so as to lock it against rotation. The vehicle occupant is thereby securely restrained against excessive forward movement.
The locking pawl ideally comes into engagement quickly and has a high load bearing capacity to withstand the very high-localized stresses, which occur around the toothed ratchet wheel and the tip of the pawl. The stresses can be accommodated by using steel components, particularly for the ratchet wheel and the pawl. These are strong and generally not prone to distortion but this adds significantly to the cost and weight of the seat belt retractor and additional assembly time is required. Aluminum or zinc components are preferred because of the weight and cost advantage.
Alternatively additional locking strength can be introduced by using more than one tooth on the pawl. Also two pawls may be attached at opposite ends of a lock bar so as to lock the retractor spool at both ends. A disadvantage of using more than one tooth on the locking pawl is in ensuring that the teeth on the pawl engage fully with the teeth on the ratchet wheel. It is known for example from U.S. Pat. No. 5,593,105 to use three teeth on a locking pawl and for the teeth to have the same pitch as the teeth on the ratchet wheel. However, this requires a complex pivoting arrangement there is a tendency, under particularly high loads, for one or more of the teeth to become disengaged or partially engaged after locking has occurred due to distortion of components under load. obviously if all the teeth become disengaged then this is especially dangerous in a crash situation. Also if only one or two teeth become disengaged then the number of teeth bearing the load is reduced and the stresses in the locking components are relatively increased which can lead to premature failure of parts.